1. Field of the Invention
The present invention relates to a magneto-optical recording medium for recording or reproducing information therein with a laser beam by utilizing the magneto-optical effect. More particularly, the present invention relates to a magneto-optical recording medium which can increase the density of information recorded in the medium and a method for reproducing information therein.
2. Related Background Art
As a rewritable high-density recording method, attention has been directed to a magneto-optical recording medium in which magnetic domains are written on a magnetic thin layer by thermal energy of a semiconductor laser to record information, and from which the information is read using the magneto-optical effect.
Recently demand is high to increase a recording density of the magneto-optical recording medium so as to provide a larger storage recording medium has become high.
A linear recording density of an optical disc such as the magneto-optical recording medium is determined mainly by an S/N ratio of a reproduction layer, which depends considerably on a bit string cycle of a signal, a laser wavelength of a reproduction optical system and a numerical aperture of an objective lens.
Namely, once a laser wavelength .lambda. of a reproduction optical system and a numerical aperture NA of an objective lens are determined, a bit cycle f serving as a detection limit is also determined to be .lambda./2NA.
On the other hand, a track density of and optical disc is restricted mainly by cross talk. The cross talk depends mainly on a distribution (profile) of a laser beam on a surface of the medium and is represented by a same function of .lambda./2NA as the above-mentioned bit cycle.
Accordingly, in order to increase the density of information to be recorded with a conventional optical disc, the laser wavelength of a reproduction optical system needs to be shortened and the numerical aperture NA of an objective lens needs to be enlarged.
However, there are limitations as to an improvement in the laser wavelength and an improvement in the numerical aperture of an objective lens. Therefore, techniques have been developed such that a structure of a recording medium or a reading method is improved so as to increase the recording density.
For example, Japanese Laid-open Patent Application No. 3-93058 attempted to improve the recording density by using a medium having a reproduction layer and a recording layer. Namely, magnetization in the reproduction layer is aligned with one direction before reproducing information, and then the information held in the recording layer is transferred to the reproduction layer so as to decrease inter-symbol interference upon reproduction, whereby information with a cycle less than the diffraction limit of light can be reproduced.
However, Japanese Laid-open Patent Application No. 3-93058 has such a drawback that an adequate reproduction power cannot be obtained. In more detail, each bit in the recording layer is transferred to the reproduction layer and reproduced therefrom in the same size as it is recorded. Since the each bit is recorded in the recording layer with a size considerably smaller than the size of a laser spot, the resolving power will be very low for such small bits.
Further, since the magnetization in the reproduction layer must be aligned in one direction before irradiation by a laser beam, an initialization magnet for the reproduction layer needs to be added to a conventional apparatus. Therefore, the above-mentioned reproduction method has such problems that the magneto-optical recording apparatus is complicated, the cost is high and miniaturization is hardly possible.